This invention relates to applying an aluminum-containing coating to a metallic surface, and, more particularly, to a maskant that allows some regions of the surface to be coated and prevents the coating of other regions.
Nickel-base superalloy components of gas turbines are sometimes coated with aluminum and simultaneously heated to diffuse the aluminum into the surface of the article. The aluminum-rich surface is thereafter oxidized by heat treatment or in service to produce an adherent aluminum oxide scale on the surface of the article. The aluminum oxide scale is effective in inhibiting and slowing further oxidation and corrosion of the component in service. The aluminum may also be interdiffused with preexisting layers of other compositions to produce more complex diffusion aluminide protective coatings.
The aluminum-containing coating is typically applied by vapor phase deposition, chemical vapor deposition, pack cementation, above-the-pack processing, or similar techniques. In one such approach, aluminum halide gas is contacted to the component surface under conditions such that the compound decomposes to leave a layer containing aluminum deposited on the surface. The aluminum-containing coating diffuses into the surface during the deposition and any post-deposition heat treatment, producing the aluminum-enriched surface region.
It is sometimes the case in such deposition processes that a first region of the surface of the article is to be left uncoated, and a second region of the surface of the article is to be coated with the aluminum-containing material. In order to prevent deposition of aluminum from the aluminum-containing source, the first (uncoated) region of the surface of the article is physically covered with a maskant that overlies and contacts the surface of the article. The maskant prevents contact of the aluminum-containing gas to the first region of the surface. Available maskants usually include sources of Ni+2 and C+r ions in a binder complex with Al2O3 and possibly other oxide particles. These maskants are intended to prevent the coating vapors from reaching the surface of the article.
The present inventors have observed that, after removal of the maskant from the first region of the substrate surface, there may be a depletion of the aluminum content of the substrate alloy at the substrate surface to a depth of up to about 0.0005-0.002 inches. In addition to providing strengthening of the substrate through the formation of gamma prime precipitates, the aluminum forms a protective aluminum oxide that inhibits destructive oxidation of the substrate during service at elevated temperatures. The depletion in aluminum content under the maskant, even to a relatively small depth, results in a loss of oxidation resistance at the uncoated surface, and may also result in a reduction in the mechanical properties of the material due to the reduced ability to form gamma prime precipitates. The depletion in aluminum content may also adversely affect other processing modifications of the substrate surface.
There is a need for an improved approach to the aluminide coating of an article surface where some of the surface must remain uncoated.
The present invention provides an improved maskant for use in aluminiding a surface, and a method of aluminiding that utilizes the maskant. The maskant functions to prevent aluminiding of the region of the surface covered by the maskant, while at the same time substantially reducing and, ideally, eliminating depletion of aluminum from the region of the substrate surface covered by the maskant. The maskant is used in the same manner as conventional maskants.
A maskant is used in aluminiding a surface of a metallic substrate, where the metallic substrate has a substrate surface composition comprising nickel, a substrate aluminum content, and other alloying elements. The maskant includes a plurality of maskant particles, each particle having a maskant particle composition comprising a maskant metal selected from the group consisting of nickel, cobalt, titanium, chromium, iron, and combinations thereof, and a maskant aluminum content. The maskant metal is preferably nickel.
A method for aluminiding a portion of a surface, while not aluminiding other portions of the same surface, comprises the steps of providing a metallic substrate having a substrate surface and a substrate surface composition comprising nickel, a substrate aluminum content, and other alloying elements, and applying a maskant overlying a protected region of the substrate surface to produce a masked substrate surface having an exposed region and the protected region. The maskant comprises a plurality of maskant particles, each particle having a maskant particle composition comprising a maskant metal selected from the group consisting of nickel, cobalt, titanium, chromium, iron, and combinations thereof, and a maskant aluminum content. The method further includes contacting an aluminum-containing material to the masked substrate surface, whereby aluminum deposits on the exposed region and does not deposit on the protected region.
The maskant particles of the maskant may be of substantially the same composition as the substrate surface. The maskant particles may instead be primarily the maskant metal and aluminum, with the aluminum content preferably about that of the substrate, but without other expensive alloying elements found in the substrate that have no function in the maskant. In another alternative, the aluminum content of the maskant particles is as high as the final aluminum content of the coating to be applied in the unmasked areas. Intermediate aluminum contents are also operable.
The maskant particles may be the only type of metallic particles present, or there may be conventional particles such as nickel particles having substantially no aluminum.
The maskant particles may be distributed throughout the maskant, or they may be preferentially concentrated at the surface of the maskant that lies adjacent to the substrate surface. In the latter case, the maskant particles may be applied directly to the surface of the substrate or may be preferentially positioned at the surface of an applied maskant layer.
The maskant particles reduce the reactivity of the maskant for the aluminum in the substrate, to inhibit depletion of the aluminum from the protected portion of the substrate contacted by the maskant, while retaining the ability of the maskant to react with aluminum externally introduced in the aluminiding process This latter ability is important to prevent the aluminum introduced by the aluminiding process from reaching and reacting with the protected portion of the substrate surface.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.